Conventionally known vitamin D.sub.3 derivatives include natural metabolites, such as 25-hydroxyvitamin D.sub.3 and 1.alpha.,25-dihydroxyvitamin D.sub.3, and synthetic analogues thereof, such as 1.alpha.-hydroxyvitamin D.sub.3,1.alpha.,24-dihydroxyvitamin D.sub.3, and various fluorinated vitamin D.sub.3 derivatives. Of these vitamin D.sub.3 derivatives, 1.alpha.,25-dihydroxyvitamin D.sub.3, 26,27-hexafluoro-1.alpha.,25-dihydroxyvitamin D.sub.3, etc. are known to have a potent calcium metabolism regulating activity and be useful for various osseous lesions.
Known vitamin D.sub.3 derivatives having a substituent at the 2.beta.-position include 1.alpha.,25-dihydroxy-2.beta.-fluorovitamin D.sub.3 disclosed in Japanese Patent Publication (Kokoku) No. 14303/91 and the derivatives having a substituted lower alkoxy group at the 2.beta.-position disclosed in Japanese Patent Application Laid Open (Kokai) No. 267549/86.
The in vivo metabolic pathways of natural type vitamin D.sub.3 derivatives such as the known active metabolites have been established so that there are limits of dose in administration of these vitamin D.sub.3 derivatives as drugs. The non-natural type fluorinated vitamin D.sub.3 derivatives involve complicatedness of fluorine introduction in the preparation thereof. Besides, the fluorinated derivatives are expected to be less susceptible to metabolism in a living body and be apt to retained in the body, causing side effects.
Vitamin D.sub.3 derivatives with a substituted lower alkoxy group at the 2.beta.-position have a possibility that the substituent may be cut off by in vivo metabolism because it is introduced through an ether linkage.